Encoders have been used for measuring physical quantities, such as the position and velocity of a moving body.
Encoders are classified broadly into rotary-type (hereinafter, also referred to as “rotary”) encoders and linear-type (hereinafter, also referred to as “linear”) encoders depending on the direction of movement of a moving body.
Rotary encoders are also referred to as rotational position detecting devices, for example, and detect the position (angle) and velocity (rotational velocity) of a moving body (a rotating body). By contrast, linear encoders are also referred to as linear position detecting devices, for example, and detect the position and velocity of a moving body.
Non-contact encoders are classified broadly into “magnetic (including resolvers)” encoders and “optical” encoders depending on their detection principle and the like. Magnetic encoders have characteristics of excellent environmental resistance compared with optical encoders, for example. Optical encoders have characteristics of excellent position resolution compared with magnetic encoders, for example. Furthermore, there have also been developed encoders (also referred to as “hybrid” encoders) using both magnetism and light so as to provide the characteristics of both encoders.
Furthermore, encoders are classified broadly into incremental-type (hereinafter, also referred to as “incremental”) encoders and absolute-type (hereinafter, also referred to as “absolute”) encoders depending on their position detection method and the like. Incremental encoders mainly detect the relative position of a moving body with respect to an origin position. Specifically, incremental encoders detect an origin position in advance and acquire a periodic signal, such as a pulse signal, corresponding to movement from the origin position. Subsequently, incremental encoders perform processing of integration of the periodic signal, thereby detecting the position, for example. By contrast, absolute encoders are also referred to as absolute value encoders and detect the absolute position of a moving body.
Each type of encoder among the various types of encoders described above is appropriately selected and used depending on characteristics required for intended uses. In particular, encoders play an important role in servomotors (including rotary motors and linear motors), which perform control, such as position control and velocity control, grasping a present position, for example. In other words, performance and characteristics of an encoder selected and used in a motor can influence performance and characteristics of the motor.
An optical encoder will be explained below.
As optical encoders, there have been developed encoders in which a grating formed of a plurality of slits (including reflective and transmissive) is used. Encoders using the optical grating are classified broadly into “geometrical optical” that uses light simply transmitted through or reflected from the grating and “diffraction interference optical” that uses diffraction interference light obtained by a plurality of gratings (Japanese Patent No. 3509830 and Japanese Patent Application Laid-open No. H6-347293).
The geometrical optical encoder receives light reflected by or transmitted through the slits forming the grating without causing the light to be diffracted and interfered and specifies the positional change and the like on the basis of the number of times the light is received and the like. This geometrical optical encoder has characteristics that the detection accuracy is easy to decrease as the distance (hereinafter, also referred to as a “gap g”) between one grating and another grating, a light receiving unit, or the like becomes larger when the slit intervals (hereinafter, also referred to as a “pitch p”) in the grating is made constant.
On the other hand, the diffraction interference optical encoder uses diffraction interference light obtained by a plurality of gratings and specifies the positional change and the like on the basis of the number of times the diffraction interference light is received and the like. Therefore, this diffraction interference optical encoder can improve the S/N ratio (Signal to Noise Ratio) compared with the geometrical optical encoder. Moreover, the diffraction interference optical encoder has characteristics that the detection accuracy is less likely to be affected even when the gap g is set relatively long. This means that the environmental resistance such as against impact can be improved by reducing the possibility of causing a mechanical interference between components. In this manner, the diffraction interference optical encoder is more advantageous than the geometric optics encoder.
However, in the diffraction interference optical encoder, because a diffraction interference optical system needs to be formed, the pitch p for each of a plurality of gratings (diffraction gratings) and the gap g that is an interval between the gratings are set to appropriate values. The relationship between the pitch p and the gap g restricts development and manufacturing of the encoder itself. That means that if the pitch p or the gap g is changed from an appropriate value, the quality of diffraction interference light decreases and the S/N ratio of a periodic signal to be detected decreases. On the other hand, in order to maintain the pitch p or the gap g to an appropriate value, the diffraction interference optical system needs to be designed and developed in consideration of the periodic numbers of a periodic signal, the formation position of slits, and the like in addition to the pitch p and the gap g.
Accordingly, flexibility decreases and therefore designing and development are not easy. Moreover, because adjustment is needed for each of a plurality of diffraction interference optical systems, the diffraction interference optical encoder is difficult to manufacture. Furthermore, such restrictions on the designing and development make it difficult to downsize the device itself.
The restrictions on designing, development, and manufacturing may be imposed even when one diffraction interference optical system is used for obtaining one periodic signal. However, particularly, when a plurality of diffraction interference optical systems is used for obtaining an origin signal, for example, in the case of an incremental encoder, designing, development, and manufacturing need to be performed for each of the diffraction interference optical systems, therefore, the degree of restrictions on them further increases.
For example, an optical encoder that obtains an origin signal by a diffraction interference optical system is disclosed (PCT Publication No. WO07/108,398).
This optical encoder includes a rotary slit for origin phase consisting of linear slit patterns arranged in parallel at equal pitches in a rotary disk, and a light source slit for origin phase consisting of linear slit patterns arranged in parallel at equal pitches and a fixed slit for origin phase are included in a fixed scale for origin phase.
The rotary slit for origin phase is irradiated with irradiation light from a light source through the light source slit for origin phase. Reflected light from the rotary slit for origin phase is passed through the fixed slit for origin phase and detected by a light receiving element and an origin signal is created from its detection signal.
However, in this detection method, a sharper signal needs to be obtained to obtain a highly accurate origin signal. In order to obtain a sharper signal, the area of the rotary slit for origin phase and the light receiving surface need to be increased. Accordingly, it is difficult to achieve both downsizing and high accuracy.